Screw top caps have been used for some time to seal various containers. Although many screw tops include a separate sealing gasket within the cap, there is substantial advantage to be had in producing a one-piece cap which still effectively seal the container.
Such a one piece cap is shown in the British patent 788148 (3 Aug. 1956) which includes a continuous lip within the top portion of the cap positioned to engage against the annular end face of the opening and provide a seal between the lip and the free end edge of the container with the lip curling over at its free edge. However, this cap provides a seal only against the free end edge of the container.
Australian application 15456/76 (30 Jun. 1976) discloses an alternative one-piece cap in which a annular lip extends from the inside top of the cap and engages the inner bore of a container opening so as to curl the free end of the lip in against the bore or inside surface of the opening. However, with his cap, effective sealing requires that the inside bore of the opening be of accurate and consistent dimensions. Furthermore, if aerated or other gaseous liquid is to be contained, gas pressure will tend to distort the lip and cause a seal failure.
Australian patent application 14180/83 (5 May 1983) describes a cap with two internal sealing structures. One of the structures is an annular shaped outer portion shaped to accept the outer peripheral edge of the free end of the container relying upon the pressure generated during the closing of the cap to seal against this outer edge. Further provided is an inner cylindrical lip to engage the inner bore of the container opening.